Although mobile communications instruments, such as a cellular phone and a portable terminal, have been spread rapidly in recent years, a filter used for these terminals is required for characteristics, that is low energy loss, a wide frequency band, and a small size, etc. A transmitting type surface acoustic wave (SAW) filter is in practical use, in which said filter has a single-phase unidirectional transducer as a device which fulfills these characteristics. In a single-phase unidirectional filter, a phase difference between an excitation wave and a reflective wave becomes the same phase to enhance each other in a front direction (at a forward direction), and is negated each other in an opposite direction (at a reverse direction). Therefore, the surface acoustic wave is strongly driven only at the forward direction. By this way, it is theoretically possible to realize a low loss filter less than 1 dB by opposing the direction of the unidirectional property of a transmitting electrode and a receiving electrode.
As the technology of realizing the unidirectional transducer, EWC-SPUDT and DART-SPUDT, using the asymmetrical electrode structure, are designed. Besides these filters using the asymmetry of the electrode structure, there is a natural unidirectional filter (NSPUDT: Natural Single Phase Unidirectional Transducer). The natural unidirectional filter realizes unidirectional property by using the asymmetry of a substrate crystal. Therefore, unidirectional property is realizable with the transducer having the structure called regular type inter digital transducer (IDT) structure, in which the multiple positive/negative electrode fingers, where both the electrode width and the electrode spacing are λ/4, have been arranged continuously and periodically.
Even if regular type IDT is formed on a ST-X quart crystal substrate, the surface acoustic wave generated by the excitation driving of said IDT is spread on a ST-X quart crystal substrate for the two way of said IDT, and cannot realize unidirectional property. That is to say, the natural unidirectional property shows the characteristic of the substrate, in which the surface acoustic wave is excited strongly to the uni-direction, when the regular type IDT is formed on a piezoelectric crystal substrate surface. In the surface acoustic wave transducer using this natural unidirectional substrate, since an anisotropy of the substrate itself is used, the forward directions of the transmitting-side transducer and the receiving-side transducer, cannot be opposed. If the unidirectional property cannot be opposed between the transmitting electrode and the receiving electrode, it is impossible to produce the low loss filter.
As the means to solve this problem, in Tokukai Hei 8-125484, the surface acoustic wave transducer is proposed as the electrode structure to reverse the direction of natural unidirectional property by Mr. Takeuchi etc., in which said surface acoustic wave transducer consisted of the positive and negative electrode fingers, which had the width of almost λ/8 and were arranged with the pitch of λ, and the floating electrodes, which had the width of 3/8λ and were arranged with the edge interval of almost λ/8 between said positive electrode fingers and negative electrode fingers.
The low loss filter which opposed the unidirectional property, is proposed by Mr. Takeuchi etc. in Tokukai Hei 8-204492, in which said filter used the electrode structure of reversing the direction of natural unidirectional property. In the electrode structure of reversing the direction of natural unidirectional property proposed by Mr. Takeuchi etc., when the wavelength of the surface acoustic wave is λ, the positive electrode finger and the negative electrode finger are arranged with the pitch of λ, respectively, the distance between each center of the positive electrode and the negative electrode is λ/2, the floating electrode having the width of λ/4 is formed between the positive electrode finger and the negative electrode finger, and the distance between each edge of the positive electrode finger or the negative electrode finger and the floating electrode, is formed so that it may be λ/16. Moreover, at least, the floating electrode of adjoining one pair is short-circuited (FIG. 5 in the above-mentioned patent), and it is also included when all floating electrodes are short-circuited (FIG. 6 in the above-mentioned patent).
The characteristic of the surface acoustic wave device depends on the characteristic of the piezoelectric crystal used as the substrate. It is important that an electromechanical coupling coefficient is large, and an frequency temperature characteristic is good, as the characteristic of this piezoelectric crystal. Now, langasite is attracted as the crystal which satisfies simultaneously these two characteristics. When langasite is described in (φ,θ,ψ) by Euler-angles viewing, the langasite being in the range of −5°≦φ≦5°, 135°≦145°, and 20°≦ψ≦30°, has the electromechanical coupling coefficients being 0.3% -0.4%, and the frequency temperature characteristic showing the secondary dependency, and the summit temperature existing near a room temperature. An electromechanical coupling coefficient is about 3 times of ST quart crystal. Moreover, the secondary temperature coefficient in the frequency temperature characteristics is about 2 times of quart crystal, and this value is very good. Therefore, langasite is expected to be applied to the low loss surface acoustic wave filter.
Langasite single crystal which is in the above-mentioned ranges by Euler-angle viewing, has NSPUDT characteristic, and for realizing the low-loss filter using this substrate, it is necessary that the electrode structure where the direction of unidirectional property counters in the transmitting electrode and receiving electrode, is constituted. Therefore, when regular type IDT is used for the transmitting electrode, wherein the multiple positive/negative electrode fingers, where both the electrode width and the electrode spacing are λ/4, have been arranged periodically and continuously, the structure, in which unidirectional property is inverted, has to be used in the receiving electrode. However, in the electrode structure proposed by Mr. Takeuchi etc., the optimum unidirectional inversion cannot be realized on the langasite substrate, and the request for realizing of the low loss filter cannot be answered.
This invention was made in view mentioned above, and aims to offer the surface acoustic wave device, which is enable to comprise the much low loss transmitting type surface acoustic wave (SAW) filter.